1. Field of the Invention
The present invention generally concerns a demodulator of phase modulated carrier pulse signals of the phase lock loop type.
In data transmission systems of the kind concerned by the invention, the digital signals transmitted by the system are represented by N significant values or codes of the phase of the carrier of pulses of said carrier having a given width, the said phase values being those of the carrier at a given time within said width with respect to a reference signal of the same frequency as the transmitted carrier. For fixing one's idea, for a transmission rate of 1600 bauds, the carrier pulses have a duration of 10.sup.6 /1600 = 625 .mu.s and the frequency f of the carrier is 1800 Hz, whence 1/f = 555 .mu.s. Since the code is a N-ary code, the N distinct signals of the code can be respectively assigned to N characters or to group of n bits with 2.sup.n = N.
Let us assume for instance that N = 8, whence n = 3; the allotment of the phase codes to the bit groups can be the following:
______________________________________ .phi..sub.0 = 0 0 0 0 .phi..sub.4 = .pi./2 1 0 0 .phi..sub.1 = .pi./4 0 0 1 .phi..sub.5 = 5.pi./4 1 0 1 .phi..sub.2 = .pi./2 0 1 0 .phi..sub.6 = 3.pi./2 1 1 0 .phi..sub.3 = 3.pi./4 0 1 1 .phi..sub.7 = 7.pi./4 1 1 1 ______________________________________
More precisely, this phase modulated carrier pulse system is operated as a differential system, i.e. a given character is not represented by a determined phase but by the phase shift between the phase code assigned to the character and the phase code assigned to the preceding character. Otherwise speaking, only phase differences or shifts are transmitted.
For decoding the phase codes, it is necessary to reconstitute the reference carrier from the randomly occuring phase codes. This is not directly possible and in fact a multiplied reference signal having a frequency equal to N times the carrier frequency is first reconstituted then this multiplied reference signal is divided by N. Once the reference carrier has been reconstituted, decoding consists in comparing the phase of the carrier in each pulse to the phase of the reconstituted carrier.
The principal object of the present invention is to provide an improved phase lock loop system for reconstituting the carrier frequency in phase modulated carrier pulse transmission systems.
Another object of the invention is to provide a phase lock loop system for reconstituting the carrier frequency in phase modulated carrier pulse transmission systems which has a very high speed of loop lock up. To fix one's idea, the loop transient settling time is of the order of 10 milliseconds.
2. Description of the Prior Art
The simplest way for reconstituting the carrier frequency is to multiply the carrier in each pulse by N, then to apply the multiplied frequency signals to the phase detector of a phase lock loop system comprising a low-pass filter, an integrator and a voltage controlled oscillator controlled by said low-pass filter and whose output is fed back to the second input of the phase detector and which also supplies the output multiplied reference signal.
A phase lock loop digital circuit which provides a very high speed of loop lock up while maintaining optimum loop bandwidth during the steady state lock up operation is disclosed in U.S. Pat. No. 3,795,870 issued Mar. 5, 1974. This phase lock loop circuit employs a frequency/phase detector circuit which operates in the frequency detector mode when the loop is unlocked to control a speed up circuit to slew a local oscillator to the lock condition in a time order of magnitude faster than with standard forms of phase detectors. The frequency phase detector thereafter functions in the phase detector mode to maintain the loop in the steady-state lock conditions. When the speed up circuit brings the loop to the locked condition, the bandwidth of the phase lock loop is at a high value to provide a fast loop transient settling time. At the time the locked loop settles into its steady state condition, the loop bandwidth is lowered to the optimum value for good noise performance.
Such a phase lock loop circuit cannot be used in phase modulated carrier pulse signal transmission systems since it assumes that synchronous pulses having precise repetition frequency can be derived from the signals whose phases or frequencies have to be compared. This can readily be done for the reference signal by selecting as VCO a variable frequency pulse generator. But as regards to the phase modulated carrier pulse input signal, since in each pulse, the instants of passage through zero of the carrier randomly vary within the pulse width, it is not possible to derive from the passing through zero instants a train of synchronous pulses.
Further during transmission, the signal in base band may be frequency transposed in modulators and demodulators and it may arise than the two frequency transpositions do not result in the reconstitution of the same base band carrier frequency. The carrier frequency may thus deviate by several Hertz from its nominal value. Finally, phase jumps of the carrier occur when passing from one pulse to the following. From these two grounds, there results a jig (that is a certain amount of filter) of the passing through zero instants.